Also, there would have to be a mechanism for plugins to add to the EHLO response, to advertise the capability.
The recent code that I added to enable AUTH support does this. Check out Qpsmtpd/SMTP.pm about line 171. In general, the idea is to grab $self->transaction->notes('capabilities') and append your line (a technique that I cribbed from Guillaume Filion's first pass on the AUTH module). There is no reason this cannot be handled with the existing ehlo hook, though.
John
